UC Berkeley Chancellor Robert Berdahl issued that ultimatum late yesterday after a group of unidentified students admitted to the Daily Californian, the student newspaper, that they had taken the 10-foot-tall costume of an antic redwood tree in a burglary at Stanford on October 16.

The students, who called themselves "The Phoenix Five," told the campus newspaper they were holding the bulky, 45-pound costume at an "undisclosed location."

However, they promised they would return the missing mascot outfit to Stanford unharmed in time for the 101st Big Game on November 21 at Cal's Memorial Stadium, but they didn't give a specific date.

In the prankish spirit that surrounds the traditional football rivalry between the schools, one of the students said the tree was going back to Stanford "even though he knows that Stanford is a bunch of weenies."

The students said they had "freed" the costume from the Stanford Band Shak , home of the university's notoriously irreverent band, because Stanford lacks the diversity, spirit and work ethic that make "Cal a superior university and Stanford an inferior one."

However, Chancellor Berdahl was in no mood to treat the filching of the costume, valued at $1,000, as mere collegiate mischief.

Calling the students' action "outright theft," Berdahl warned in a brusquely worded statement that if the Tree isn't back in Stanford hands by tomorrow midnight, "Oski will not appear at Saturday's Cal football game (vs. Oregon State at Corvallis, Ore.) and will not appear again until the Tree is returned."

The chancellor did not say if the university would seek to prosecute the students if their identities become known, but police at both campuses say they view the burglary as a felony.

At the same time, however, Marc Wais, Stanford's dean of student affairs, repeated a previous pronouncement that Stanford would be satisfied if the costume is returned soon to Chris Henderson, the Stanford junior who is this year's Tree.

Henderson, in a brief statement he released yesterday to the media, aimed a few jibes of his own at the Cal thieves for a mock letter they released in the name of the Tree. He urged the Cal students to "stop talking to a pile of fabric and show a little backbone."